The ability of retroreflective sheeting to have the same observable retroreflective brightness regardless of the angular orientation of the sheeting on the sign has long been a desirable feature in prismatic retroreflective sheeting. This property is hereinafter referred to as “rotational insensitivity”. Early retroreflective sheeting made with glass bead technology had this feature. By contrast, the performance of prismatic retroreflective sheeting tends to be strongly sensitive to rotation in the plane of the sheeting. Consequently, if an observer directed a flashlight beam at such a retroreflective sheet while the sheet rotated, the sheet would appear to the observer to darken or lighten at least every 180° as the array of cube corner prisms on the sheet retroreflected a greater or lesser percentage of the flashlight beam. There are a number of applications of such retroreflective sheeting where it is applied in a variety of orientations (such as on automobile license plates, road cones, retroreflective strips on trucks, high visibility reflective tape, etc.) and consequently where rotational insensitivity is very much needed. There are also situations where different pieces of sheeting are placed next to each other, such as cut out letters on signs, a sign using more than one piece of sheeting, or two signs mounted next to each other. In such situations it is undesirable to introduce visual contrast between the pieces.
Various techniques have been proposed and used to minimize rotational sensitivity. One of the most common involves dividing the arrangement of cube corner prisms into adjacent groups of close-packed arrays of cube corner prisms referred to as “tiles”. Each tile typically contains prism pairs with prisms in two opposite orientations. Different tiles can have prism pairs oriented differently, so that the combined effect of all the tiles has less sensitivity to rotation. However, the manufacture of such tiled arrays is more difficult than non-tiled arrays. Additionally, the different retroreflective patterns generated by the different tiles generate an undesirable visual contrast between the tiles when the sheeting is viewed at close distances. Other techniques include using cubes on laminae having sideways canting of the optical axes with alternating sign, in such a way as to reduce orientation sensitivity. While this technique moderates rotational sensitivity, the resulting product is still likely to be undesirably sensitive to rotation.
More recent proposed solutions utilize an assembly of multi-direction retroreflective elements. In one approach, assemblies of five, seven or more elements share two common base edges and a common vertex. This approach can provide an array of prisms having ten or more orientations, which in turn can provide good rotational insensitivity. It also avoids all the work involved in tiling, as well as the visual contrast between tiles which can be seen when the sheeting is viewed from a sufficiently close distance.